The tech "Planetary Invasion" is the tech which you need to invade enemy planets. Unfortunately, in the current build it is far too cheap to research and is too early on the tree.
It needs to be more expensive and have a single prerequisite (like Galactic Warfare), because as it stands, you can rush the tech in less than 8 turns, buy a single invasion transport, and trounce the enemy before they have the opportunity to build even basic craft. This problem is exacerbated by the fact we're stuck on these small maps, and Human players (Here's looking at you, DARCA, you bastard :3) exploit this ruthlessly.
There really can be no debate; as it stands, you either focus on PI immediately after the game starts, or you lose.